When building or renovating a steel roof, choosing the correct size of Z purlin is one of the most important steps. These metal supports play a key role in holding up the roof sheeting and ensuring your structure remains safe and secure.
In this article, we’ll guide you through how to choose the right size of Z purlin for your roof by considering the span, load, and intended use. You’ll also get helpful tips from our construction experts at Swan Swee Construction on how to make the best choice for your project.
What Are Z Purlins?
Z purlins are horizontal beams made from cold-formed steel. They are shaped like the letter “Z” and used to support the roof sheets or cladding in metal buildings. Z purlins are placed between the main steel frames and fixed to the rafters or supporting structures.
They come in different sizes, thicknesses, and lengths, and choosing the correct one depends on how much weight it needs to carry and how far it needs to span.
At Swan Swee, we now offer Z purlins featuring ZM310 coating, which provides superior corrosion resistance compared to traditional galvanised steel. This makes it ideal for buildings in humid or coastal environments.
Understanding Z Purlin Sizes
Z purlins are commonly available in depths like 150 mm, 175 mm, and 200 mm, with varying flange widths and thicknesses, usually around 1.9 to 3.0 mm. The steel is often galvanised to resist rust and corrosion.
With the new ZM310 coating, some of the commonly available Z purlin sizes include:
- Z15019 (150 mm web height, 1.9 mm thick)
- Z22524 (225 mm web height, 2.4 mm thick)
- Z35030 (350 mm web height, 3.0 mm thick)
These are precision-formed sections available in various lengths and can be customised with pre-punched holes.
The size you need will depend on your roof design, spacing between purlins, and the total load the roof will carry.
Factors That Affect Z Purlin Sizing
1. Roof Span and Purlin Spacing
The wider the span between supports, the larger and thicker your Z purlin needs to be. For example, a span of 4 metres may require a smaller section than a 6-metre span. If purlins are spaced closer together, smaller sections can be used.
Always refer to a manufacturer’s span table to check which Z purlin size suits your design.
2. Roof Load
There are several types of loads to consider:
- Dead Load: The weight of the roof itself, including sheets and insulation
- Live Load: Temporary weight, such as workers walking on the roof
- Wind Load: Pressure or uplift caused by wind, especially in open areas or high buildings
- Snow Load (if applicable): In colder regions, the weight of accumulated snow
Each load adds to the stress on your Z purlin, so it’s important to calculate this correctly.
3. Roof Slope and Bracing
Z purlins are typically used in roofs with a slope of up to 25 degrees. If your roof has a steeper pitch, you may need extra support.
You should also consider installing anti-sag bars, cleats, and bracing systems to help support the purlins and prevent twisting or bending.
With ZM310-coated purlins, you also have the option to include pre-punched holes in the web, flange, or lip to accommodate bolts and fasteners— minimizing on-site drilling and enhancing installation efficiency. Standard hole types include 14 mm and 18 mm round holes, or 14×22 mm and 18×22 mm slotted holes.
Using Span and Load Tables
Span and load tables are useful tools that show you which Z purlin sizes are suitable for different spans and loads. Many steel suppliers and websites provide these resources, or you can use online calculators for quick results.
These tables will also show the maximum deflection allowed. As a general rule, deflection should not be more than span/180 for roofing applications.
Step-by-Step Guide to Choosing the Right Z Purlin
- Measure your roof span, slope and how far apart you will place the purlins
- Estimate the total load (dead, live, wind, and snow if needed)
- Refer to a span table to find a few suitable Z purlin profiles
- Check the load table to ensure the purlin can support the expected weight
- Review deflection limits to ensure the section is safe
- Confirm bracing requirements based on the design and the supplier’s guidelines
- If using ZM310-coated purlins, specify if you need pre-punched holes and indicate your preferred hole type and placement. Custom punching is available to suit different installation layouts.
Tips for Installing Z Purlins
- Make sure Z purlins are properly aligned and level during installation
- Use the correct bolts and fixings through pre-punched holes
- Where longer spans are needed, join Z purlins with end laps or sleeves
- Always check for corrosion or damage during regular maintenance
Example: Choosing a Z Purlin for a 6m Span
Let’s say you are building a steel roof with a 6m span and expect a moderate load. According to standard span tables, a Z purlin with a depth of 175 mm and a thickness of 1.9 mm may be suitable, depending on spacing and wind conditions.
By checking the load table, you can confirm if this section meets the required capacity. You may also need to add anti-sag support to prevent bending.
If you’re using the ZM310 Z15019 or Z35030 section, you’re likely to achieve greater corrosion resistance and maintain long-term structural integrity—ideal for outdoor or industrial environments.
Expert Z Purlin Advice from Swan Swee Construction
Choosing the right Z purlin size is key to building a strong, reliable, and long-lasting steel roof. By considering your roof’s span, load, slope, and bracing needs, you can make a confident decision. Always refer to trusted span and load tables, or use an online calculator if needed.
Our latest ZM310-coated Z purlins come in a wide range of sizes and are fully customisable with pre-punched holes for faster and more accurate installation. At Swan Swee Construction, we help clients across Singapore with professional Z purlin selection and installation. Whether it’s a new roof or an upgrade, our expert team ensures every steel structure we build is safe and secure.
Contact us today to learn more about our ZM310 coating purlin options and how we can support your next construction project.
